The present invention relates to lithium secondary batteries that use an ionic liquid as a flame-retardant nonaqueous electrolytic solution. More specifically, the present invention relates to lithium secondary batteries that use a nonaqueous electrolytic solution that contains bis(fluorosulfonyl)imide anions.
Lithium secondary batteries are small, light chargeable batteries with a large storage capacity per unit volume or unit weight, and are used in a wide range of devices, including cell phones, laptop personal computers, personal digital assistances (PDAs), video cameras, and digital cameras. Lithium secondary batteries have thus become indispensable for various small-sized, light-weight portable devices having relatively large power consumption. Development is also underway for middle-sized or large-sized lithium batteries for installation in electric bicycles and electric automobiles, and the development of these lithium batteries is expected to provide a means to reduce the environmental load.
Conventionally, polar nonprotonic organic solvents that easily dissolve lithium salts and that do not easily undergo electrolysis have been used as the nonaqueous solvents for the nonaqueous electrolytic solution of lithium secondary batteries. However, there is a serious problem in battery safety, because these organic solvents have very low flash points, and may cause fire or explosion by the heat of overcharge or shorting. The safety problem has not been more important than it is today in the face of the urgent need for the development of large-capacity and high-output lithium secondary batteries in response to the development of smaller and lighter electronic devices and the development of electric automobiles. Use of a flame-retardant ionic liquid for the nonaqueous electrolytic solution of lithium secondary batteries has thus been investigated in many studies.
Specifically, an ionic liquid containing bis(fluorosulfonyl)imide anions (FSI anions) as an anionic component has lower viscosity than other ionic liquids. Further, this type of ionic liquid is nonflammable, and provides high energy density and high voltage while maintaining high performance even in high-rate charging and discharge. Use of this ionic liquid as the solvent of the nonaqueous electrolytic solution can thus provide a lithium battery having high safety (Patent Document 1).
However, the lithium batteries using a combination of common graphitized carbon electrodes and the ionic liquid have higher internal resistance than the organic solvent batteries, and the output characteristics are poor. Further, the lifetime characteristics tend to be low for reasons attributable to, for example, deposition of the lithium and decomposed materials on the negative electrode surface. For these reasons, the performance levels are not sufficient for practical applications.
Lithium secondary batteries using a flame-retardant ionic liquid have thus been investigated by way of developing new battery constituent materials and examining additives to improve battery performance. See JP-A-2007-207675.